1. Field of the Invention
The present invention relates to a process for producing a semiconductor device such as a photovoltaic element including a solar cell, a photosensor, or the like.
2. Related Background Art
For the production of a solar cell which is categorized to a photovoltaic element, there are known various production processes. For instance, in the case of producing a solar cell having a photoelectric conversion semiconductor layer composed of a non-single crystalline semiconductor material (this solar cell will be hereinafter referred to as xe2x80x9cnon-single crystalline series solar cellxe2x80x9d), a plasma CVD (chemical vapor deposition) process has been widely adopted on an industrial scale. The non-single crystalline semiconductor material herein is meant to include an amorphous semiconductor material, a microcrystalline semiconductor material, a polycrystalline semiconductor material, and the like.
For the non-single crystalline series solar cell to be used as a sunlight power generation source, it is basically required that the photoelectric conversion semiconductor layer has a large area, the solar cell has excellent and stable solar cell characteristics and it exhibits a high photoelectric conversion efficiency, and the solar cell can be mass-produced In order to produce such non-single crystalline series solar cell by means of a plasma CVD process so as to satisfy these requirements, it is necessary to have due consideration so that as the photoelectric conversion semiconductor layer, a large area homogeneous semiconductor film having a uniform thickness and which excels in electrical, optical and photoconductive characteristics, mechanical characteristics, fatigue resistance upon repeated use, and use environmental characteristics can be repeatedly formed at a high speed and with a good reproducibility.
Separately, there have been proposed a variety of sunlight power generation systems using solar cells having such configuration as above described. Such sunlight power generation system typically comprises a unit in which a plurality of solar cell modules [comprising a plurality of solar cells (photovoltaic elements) electrically serialized while being sealed by a sealing material] are electrically connected with each other in series connection or parallel connection so as to obtain a desired electric current and a desired voltage. In this case, it is important that neither disconnection nor short are not occurred in each solar cell module and that all the solar cell modules are uniform as much as possible with respect to their output voltage and output electric current For this purpose, at a stage of forming the respective solar cell modules, it is an important factor that the semiconductor layers used therein, which are an utmost decisive factor of dominating the characteristics of the solar cell module, are made to be uniform in terms of the characteristics. Besides, in viewpoints of making it easy to design a solar cell module and simplifying the fabrication process of a solar cell module, it is important to make it possible to efficiently form a homogeneous semiconductor film having a uniform property over a large area, where this situation leads to improving the productivity of a solar cell and diminishing the production cost thereof.
Incidentally, the semiconductor layer of a solar cell (a non-single crystalline series solar cell) has a semiconductor junction such as p-i-n (or n-i-p) junction, p-n (or n-p) junction or the like. For instance, in the case where the semiconductor layer comprises an amorphous silicon (a-Si) material and has a p-i-n junction structure, the p-i-n junction structure comprises, for example, an n-type a-Si semiconductor layer, an i-type a-Si semiconductor layer and an p-type a-Si semiconductor layer stacked in this order, where the n-type a-Si semiconductor layer may be formed by subjecting a gaseous mixture of a film-forming raw material gas such as silane (SiH4 or the like) and an n-type dopant-supplying compound such as PH3 to glow discharge decomposition, the i-type a-Si semiconductor layer may be formed by subjecting said film-forming raw material gas to glow discharge decomposition, and the p-type a-Si semiconductor layer may be formed by subjecting a gaseous mixture of said film-forming raw material gas and an p-type dopant-supplying compound such as B2H6 to glow discharge decomposition.
For the production of a non-single crystalline series solar cell having such semiconductor junction, there are known methods using a multi-chambered plasma CVD apparatus comprising a plurality of film-forming chambers communicated with each other, where a semiconductor layer having a desired conduction type is formed by each film-forming chamber to form a semiconductor layer having a stacked structure with a desired semiconductor junction. As a representative example of such plasma CVD apparatus, there can be mentioned a roll-to-roll type plasma CVD apparatus for continuously forming a deposited film on an elongated substrate as disclosed, for instance, in U.S. Pat. No. 4,400,409. The roll-to-roll type plasma CVD apparatus described in this document comprises a plurality of glow discharge regions (film-forming spaces in other words) provided in a plurality of processing chambers communicated with each other. In said document, there is described that semiconductor elements having a semiconductor junction can be continuously formed by continuously transporting an elongated flexible substrate having a desired width in the longitudinal direction along a route of sequentially passing through the glow discharge regions while forming a semiconductor layer of a desired conduction type on said substrate by each glow discharge region.
In this apparatus, each glow discharge region (that is, each film-forming space) is provided in the corresponding film-forming chamber whose inside can be maintained in a vacuumed state. In order to prevent film-forming raw material gas including doping gas (dopant-supplying gas) used for the formation of a deposited film as a semiconductor layer in each glow discharge region (film-forming space) from being diffused or contaminated into the glow discharge region situated next thereto, a gas gate is provided between each adjacent film-forming chambers. The gas gate comprises a slit-like separation passage through which adjacent film-forming chambers are communicated, where separation gas such as Ar gas, H2 gas or the like is flown into the separation passage to form a gas flow of the separation gas whereby the adjacent film-forming chambers are isolated one from the other.
The film-forming method using the above-described roll-to-roll type plasma CVD apparatus is suitable for mass-producing functional deposited films or semiconductor devices such as photovoltaic elements including solar cells. However, there are such disadvantages as will be described below.
That is, because the space for forming a p-type or n-type semiconductor layer is substantially isolated from the space for forming an i-type semiconductor layer as above described, it is possible to prevent a dopant in the gaseous state used in the former space from being contaminated into the latter space.
However, for instance, at the time of forming an i-type semiconductor layer on a previously formed n-type semiconductor layer or after the i-type semiconductor layer is formed on the n-type semiconductor layer, there is an occasion in that for example phosphorous element (P) as the dopant in the n-type semiconductor layer is thermally diffused into the i-type semiconductor layer, where the n-i junction is weakened.
A solar cell whose photoelectric conversion semiconductor layer has a semiconductor junction including such weakened n-i junction has inferior initial characteristics such that the open-circuit voltage (Voc) and fill factor (F.F.) are insufficient and therefore, the initial photoelectric conversion efficiency is insufficient.
Besides, in the case of a solar cell having a photoelectric conversion layer with a p-i-n junction formed in accordance with the film-forming method using the foregoing roll-to-roll type plasma CVD apparatus, even when the solar cell has a satisfactory initial photoelectric conversion efficiency, it is liable to have such disadvantages as will be described in the following. That is, when the solar cell is continuously used under severe environmental conditions outdoors, there is a tendency in that the dopant in the p-type semiconductor layer or the n-type semiconductor layer is gradually thermally diffused into the i-type semiconductor layer to deteriorate the characteristics of the solar cell, where the photoelectric conversion efficiency of the solar cell is eventually deteriorated. Thus, the solar cell is insufficient in terms of the reliability.
As previously described, the film-forming method using the foregoing roll-to-roll type plasma CVD apparatus is suitable for mass-producing solar cells. However, there are still subjects to be improved in order to stably and efficiently mass-produce highly reliable solar cells having uniform solar cell characteristics and which exhibit a satisfactory photoelectric conversion efficiency which is hardly deteriorated even when continuously used under severe environmental conditions outdoors over a long period of time at a reasonable production cost.
Now, in the case where a plurality of solar cell modules [comprising a plurality of solar cells (photovoltaic elements) electrically serialized while being sealed by a sealing material] are electrically connected with each other in series connection or parallel connection into a unit, the solar cell module whose output electric current or output voltage is minimum becomes a rate-limiting factor to dominate the characteristics of the unit.
In this connection, it is very important to improve not only the average characteristics of all the solar cell modules involved but also variations among the solar cell modules in terms of the characteristics.
For this purpose, at a stage of forming the respective solar cell modules, it is necessary for the semiconductor layers used therein, which are an utmost decisive factor of dominating the characteristics of the solar cell module, to be made such that they are complete with respect to the characteristics. In addition, in order to reduce the production cost, it is also necessary to make the semiconductor layers have fewer defects so that neither disconnection nor short are occurred in the solar cell module, whereby the yield is improved.
In view of the above situation, there is an increased demand for improving the process for producing a photovoltaic element (a solar cell) by way of continuously forming a plurality of semiconductor layers on an elongated substrate (or a web substrate) which is continuously moving by means of plasma CVD, so that each of said plurality of semiconductor layers can be continuously and uniformly formed on the elongated substrate so as to have uniform characteristics and fewer defects over the entire of the elongated substrate.
The present invention has been accomplished in view of the foregoing technical subjects to be solved in the prior art.
An principal object of the present invention is to eliminate the foregoing problems in the prior art and to provide an improved process which enables one to efficiently produce a highly reliable semiconductor device having improved output characteristics.
The semiconductor device in the present invention includes a photovoltaic element including a solar cell, a photosensor, and the like.
Another object of the present invention is to provide a process for producing a highly reliable semiconductor device having a photoelectric conversion member and which has improved output characteristics at an improved yield, characterized in that a plurality of semiconductor layers to constitute said photoelectric conversion member are continuously formed so that each of said plurality of semiconductor layers has improved uniform characteristics and fewer defects.
A further object of the present invention is to provide a process for producing a highly reliable semiconductor device having a photoelectric conversion member formed by sequentially forming a p-type or n-type semiconductor layer composed of a non-single crystalline silicon series semiconductor material, an i-type semiconductor layer composed of a non-single crystalline silicon series semiconductor material, and an n-type or p-type semiconductor layer composed of a non-single crystalline silicon series semiconductor material by means of plasma CVD, characterized in that at least one i-type semiconductor layer as said i-type semiconductor layer is formed in a discharge chamber (or a film-forming chamber) by means of VHF (very high frequency) plasma CVD provided with a cathode electrode using a silicon atoms-containing raw material gas, wherein a VHF power of a wattage which is two times or less that of a VHF power required for decomposing 100% of said silicon atoms-containing raw material gas is applied to said cathode electrode.